In the case of detecting ions in a sample solution, there is used an ion-selective electrode method in which an ion-selective electrode is used. The ion-selective electrode is an electrode for detecting a potential difference corresponding to ion activities (i.e., thermodynamically compensated concentration) of anions and cations in a solution, and is provided with an ion sensitive (or responsive) member (membrane) which is sensitive to a specific ionic species.
Most ion-selective electrodes of conventional types require internal solutions, and, therefore are large in size and require ample quantities of sample solutions to be measured. In contrast with this, there is another type of ion-selective electrode, i.e., an all-solid-types which employ solid membranes such as solid electrolytes in their ion sensing portions (i.e., sensitive membranes) and which do not require any internal solution. In the latter types, there are two methods for detecting electric potentials: in the first one, a lead wire is directly connected with the responsive membrane to read the electric potentials; and, in the second one, the lead wire is connected with the sensitive membrane through a solid internal electrolyte layer employed in place of the internal solution to read the electric potentials, the layer being called an ionic bridge. However, the first one is poor in response time and stability in operation, which makes it difficult to obtain reliable data.
P. Fabry et al. have disclosed, in their paper "Internal Ionic Bridge for Ion Solid State Sensors, proc. of 2nd Int. Meeting on Chemical Sensors, Bordear 1986, 5-02, pp. 473-476", an ion-selective electrode in which: the NASICON, which is a solid electrolyte, is used as a solid state membrane; and, polyethylene oxide (PEO) doped with Na-ion and M-(i.e., Cu, Ag-)ion is used as an ionic bridge. As a result, the following electrochemical equation chain is clarified. ##STR1##
P. Fabry et al. have further disclosed, in their paper "NASICON, an ionic conductor for solid-state Na.sup.+ -selective electrode, Sensor and Actuators 15 (1988), pp. 33-49", a Na-ion sensor cell assembly, which is constructed of: a NASICON pellet, a lead wire made of Ag or Ag-Cd alloy, and a molten-salt type ionic bridge interposed therebetween, the ionic bridge being bonded to layers of opposite sides disposed thereto through melting/cooling operations. The ionic bridge forms an ion-conductive intermediate layer through which ions are transmitted from a solid state membrane to an internal metallic conductor (which forms a lead electrode or internal electrode). Ionic conductivities of Na- and Ag-ions are realized in the ionic bridges of AgCl-NaCl type or of AgI-NaI type, which show maximum deviations of 20 mV or 40 mV each.
The applicant of the present invention has disclosed, in Japanese Patent kokai Publication No. Hei 5-80019 (1993), an ion-selective electrode in which an ionic bridge layer is prepared by having an electrically conductive resin composition doped with at least one of salt, solid solution and solid electrolyte.